1. Technical Field
This disclosure relates to a surface-emitting laser, a surface-emitting laser array, an optical scanning device including the surface-emitting laser or the surface-emitting laser array, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Image forming apparatuses including a laser as a light source are widely used for electrophotographic recording. A typical image forming apparatus for electrophotographic recording includes an optical scanning device that scans the surface of a photosensitive drum with a light beam emitted from a light source and deflected by a deflector to form a latent image on the surface of the photosensitive drum.
Such an optical scanning device includes an optical system including a scanning lens. If the polarization state of light entering the optical system is unstable, the quality of an image output from the image forming apparatus may be degraded.
To prevent or reduce this problem, various methods for controlling the polarization state of light emitted from a surface-emitting laser have been proposed. In a first method, which is expected to be the most promising method, an inclined substrate is used (see, for example, patent document 1 and non-patent document 1).
In a second method, anisotropic stress is applied to the active layer (see, for example, patent documents 2 and 3).
In a third method, a selective oxidation layer having a rectangular or oval shape is used (see, for example, patent documents 4 through 6). In a fourth method, a selective oxidation layer with uneven thickness is used (see, for example, patent document 7).
However, the first method lacks stability. With the first method, for example, the polarization state of light becomes unstable when the temperature of the laser becomes high or the laser is operated for a long period of time. With the second method, the directions in which wires can be pulled out are limited and isotropic dry etching cannot be used for mesa formation. This in turn increases the costs of the laser. The third method adversely affects the shape (cross-sectional shape) of a light beam. With the fourth method, metal organic chemical vapor deposition (MOCVD) cannot be used for the production of the laser. This in turn increases the production costs.
[Patent document 1] Japanese Patent No. 4010095
[Patent document 2] Japanese Patent No. 3606059
[Patent document 3] Japanese Patent Application Publication No. 2006-13366
[Patent document 4] Japanese Patent No. 3799667
[Patent document 5] Japanese Patent No. 3551718
[Patent document 6] Japanese Patent No. 2891133
[Patent document 7] Japanese Patent No. 3800852
[Non-patent document 1] T. Ohtosh, T. Kuroda, A. Niwa, and S. Tsuji; “Dependence of optical gain on crystal orientation in surface-emitting lasers with strained quantum wells”, Appl. Phys. Lett. 65(15), 10, October 1994